Since you are replacing an existing radio the labor should be an hour or less, I should think. I'm assuming they would be using your existing antenna and dc wiring. Before you pay someone, however, you might look into a self install. You might be surprised at how easy it is to do, particularly if you are only replacing your 'head' unit.

Also, if you're replacing it with an aftermarket unit, I'd suggest picking up a Toyota-specific wiring harness to make it an easier swapout. You can usually get them at your local car stereo store or find them on scamazon. Look for the 'Metra' brand, they make them for pretty much any modern vehicle.

edit- If you really don't want to get into it yourself, most car audio shops will do the install for little or nothing if you buy the new unit from them. Just check around your area to see what they offer.